1. Field of the Invention
The present invention relates to a method and device for controlling handover message transmission power in a mobile communication system. More particularly, the present invention relates to a method and device for controlling transmission power of a handover ranging message considering uplink of a mobile station.
2. Description of the Related Art
In general, a mobile communication system supports handover for establishing a new communication path between a Mobile Station (hereinafter, referred to as “MS”) and a Base Station (hereinafter, referred to as “BS”) according to the change of the communication environment in order to support a seamless service.
As illustrated in FIG. 1, for the handover, the IEEE 802.16e mobile WiMAX system performs a scanning procedure (not shown) in which an MS measures signals of a serving BS and a neighbor BS, a pre-handover procedure 110 of determining a handover target BS of the MS, and a handover execution procedure 120 in which the MS resets connection with the target BS. That is, every predetermined period or whenever a received signal quality of the serving BS decreases below a predetermined level, the MS transmits a scanning request to the serving BS and acquires signal quality information of neighbor BSs. If the handover target BS is determined by the serving BS, the MS performs handover ranging for doing handover to the target BS after releasing connection with the serving BS. Here, a ranging process between the target BS and the MS gets different depending on whether an association procedure of adjusting an initial MS power value, a time difference value for synchronization, etc. with the target BS of the MS has been previously performed.
In case that the association procedure has not been previously performed, the MS selects a random ranging code from a CDMA handover ranging domain of the target BS and transmits this within a ranging interval of the target BS. At this time, if the target BS successfully receives the ranging code, the target BS transmits a ranging response (RNG_RSP) message representing a success to the MS, thereby providing uplink allocation for a ranging request (RNG_REQ) message. Or, in case that the target BS has received the ranging code but cannot use the received ranging code, the target BS transmits the MS a ranging response (RNG_RSP) message set to ‘continue’ and allows the MS to transmit a different random ranging code within a ranging interval of a next period. In case that the serving BS fails to receive the ranging code, the MS retransmits a ranging code after a predetermined time (T3) lapses.
On the contrary, in case that the association procedure has been previously performed, the serving BS transmits a Fast_ranging_IE( ) to the MS and informs of ranging code and slot information to be used in a ranging request (RNG-REQ), and the MS transmits a ranging request (RNG-REQ) message at a defined slot using the ranging code.
When performing handover ranging as above, the conventional art determines the maximum transmission power of the handover ranging message using downlink information as in Equation 1 below and then, transmits the ranging code or ranging request message at a power level below the determined maximum transmission power. Here, the maximum transmission power is a value determined on the presupposition that channel situations of uplink and downlink are symmetric.PTX_IR_MAX=EIRxPIR,max−BS_EIRP−RSS  (1)
Here, the ‘TTX_IR_MAX’ represents the maximum transmission power for a ranging message, the ‘EIRxPIR,max’ represents the maximum equivalent isotropic received power at BS, the ‘BS_EIRP’ represents equivalent isotropic radiated power of BS, and the ‘RSS’ represents an RSSI (Received Signal Strength Indicator) measured by an MS.
However, in an actual OFDMA (Orthogonal Frequency Division Multiple Access) system, a difference of interference between uplink and downlink may lead to the occurrence of asymmetric channel situations of the uplink and downlink. Accordingly, in case that it is to transmit a handover ranging message at the maximum transmission power considering only downlink as in Equation 1 above, because of the bad channel situation of the uplink, a BS may fail to successfully receive the handover ranging message. In case that the MS fails to successfully receive the ranging code as above, after the MS increases transmission power by one level and waits a response during a predetermined time (T3), the MS selects and retransmits a different ranging code. This causes an increase of a handover interruption time and, resultantly, there is a problem that a quality of a service is deteriorated. If initial transmission power is greatly set to solve the above problem, the power consumption of the MS can increase and act as great interference in a BS, thus leading to the deterioration of performance of the entire system.
Accordingly, there is a need to provide a method for controlling the maximum transmission power for handover ranging in an asymmetric situation of the uplink and downlink channels.